Interference eliminator for seismic recording systems



- P. G ILMORE I-NTERFERENCE ELIMINATOR FOR Dec. 14, 1948.

' sEIsMIc RECORDING SYSTEMS Filed Sept. 24, 1945 lnvenfor'. Ronald P.Gilmore Patented Dec. 14, 1948 l INTERFERENCE ELIMINATon Foa sEIsMIc 1RECORDING sYs'rEMs Ronald P. Gilmore, Fulton, Mo., assignor to ShellDevelopment Company, San

Francisco, Calif.,

a corporation of Delaware Application September 24, 1945Serial No.618,166

(ci. rrr-352)` 3 Claims.

This invention pertains to seismic exploration, and relates moreparticularly to a system for balancing out or neutralizing the effect ofstray exl ternal alternating current elds on the operation of seismiccircuits.

Seismic exploration normally comprises the steps of generating seismicwaves in the ground, translating these seismic waves into electricalimpulses'at a plurality of detectors or seismometers placed in contactwith the ground, transmitting the electrical impulses with suitableamplification to a recording seismograph, and recording these electricalimpulses.

Since the motions of the ground which are to be recorded are extremelysmall, the voltages produced by the detectors in response thereto arelikewise very small; When seismic exploration is undertaken in thevicinity of installations involving the use of alternating currents,such as power lines, electric railways, generator stations, etc., it hasbeen often observed that the oscillating magnetic fields surroundingthese installations are sufficiently strong to induce spuriousvoltages'in various conductors and circuits forming part of theseismograph system, such as the field coils of electro-magneticdetectors, the leads from the detectors -to the recor der, the variouscircuits of the amplifier and recorder units, etc., thus obscuring ordisiguring the true indications of the recorders.

It is therefore an object of this invention to provide a method and asystem for eliminating the interfering effects or signals induced in aseismic recording system by the oscillations of an outside magneticfield.

I-t is also an object of the present invention to provide for thispurpose a system wherein the disturbing magnetic oscillations aredetected and translated into a neutralizing alternating current havingits phase adjusted so as to be exactly opposite to that of theinterfering currents or signals induced in the seismic recording systemby said magnetic oscillations, said neutralizing current being thenintroduced into the seismic system to neutralize said interferingsignals.

These and other objects of the present invention will be `understoodfrom the following de" scription taken with reference to the attacheddrawing, giving a diagram of the circuits of the present system.

Referring to the drawing, the present invention is described in itsapplication to a conventional seismic exploration system. Such systemmay comprise any desired number of detectors, of which only detectors Iand 2 are shown for simplicity. Detectors I and 2 are connected, throughchannel amplifiers II and 22 respectively, to recording galvanometersplaced within a recorder 3, each detector, amplifier and galvanometer together with their interconnecting' circuits form.-

ing a recording channel. When such a system is operated in analternating or oscillating magnetic ileld which tends to induce thereincurrents or signals obscuring the true indications of the detectors, thee'ects of this outside field are neutralized according to the presentinvention in the following manner. A suitable conductor, such as anantenna 5, which may be mounted on the seismic truck, is used, incombination with electronic circuit means comprising, for example, atube l, to convert the interfering -magnetic oscillations, into analternating current which is l transmitted, if desired, with suitablefurther amlll plication, through a transformer 9, to the balancing orneutralizing unit 4. This unit comprises two circuits either independentor connected in parallel to the secondary Winding of the transformer 9,as shown in the drawing. The rst circuit or branch may comprise aninductive reactance such as, a coil I4 in series with a resistance I6and with a set of Helmholtz coils 24 and 26. The second branch maycomprise a capacitive reactance, such as a condenser I3 in series with aresistance I'I and with a set of Helmholtz coils 23 and 21 flxedlymounted about an axis located in the same plane as the axis of the coils24 and 26 and at right angles thereto. 'I'he eld of the coils 24 and 26is thus normal to and intersects with that of coils 23 and 21. Rotatablyarranged about a shaft 33 in -this combined Helmholtz eld is a test coil35,

The inductive reactance of the circuit comprising coils I4, 24' and 26is selected so as to have a value equal to that of its ohmic resistanceat the frequency of the interfering oscillations, which is usuallycycles, while the value of the net capacitive reactance of the circuitcomprising condenser I3 and coils 23 and 21 is selected in the samemanner with regard to its ohmic resistance. The values of the variableresistances I6 and I1 being preferably made equal, the currents in thecircuits of the coil I4 and of the condenser I3, and therefore the eldsof the Helmholtz coils 24-26 and 23-21, may therefore be made to differin phase from each other substantially by degrees, the coils 24-26 beingfurthermore displaced in space -by 90 degrees with regard to coils Underthese conditions, if the inductive reactance in one circuit isnumerically equal to the capacitive reactance of the other and each ofthese is numerically equal to the series resistance, the currents in thetwo circuits Will be exactly 90 degreesout of phase. When this conditionis satisfied, the resultantmagnetic iield produced by the sets of .theHelmholtz coils will be constant in magnitude and will revolve with anangular velocity of 60 revolutions per second. This revolving field willproduce a. 60-cycle voltage in the test coil 35 whose phase will dependonly upon the angle of orientation of the coil. By a proper adjustmentof this angle. the voltage from the test coll can be'set exactly 180degrees out of phase with any given 60-cycle disturbance, thus obtainingfrom said coil a neutralizing or balancing signal.

The coil 35 is connected, through leads 31 and 38, one of which may begrounded as shown at 44 through a battery 43to the input of as manyisolation amplifiers, such as shown at Iliand 20, as there are channelsin the system. The isolation amplifier I comprises at least oneamplifying thermionic tube 4l, whose output is applied, for examplethrough a transformer 42, to the channel amplifier Il.

The sequence of operations of the present system may therefore besummarized as follows for each channel. The oscillations of an outsidemagnetic field, caused, for example, by` an adjacent power line, inducein the circuits and conductors of seismic recording system aninterfering `signal tending to obscure the true response of the detectorto seismic waves. These outside oscillations are also picked up andconverted by the elements 5, l and 4 of the present system intoneutralizing signals of inverted phase, which are amplified by theelement I0, and introduced or applied to the channel or channelamplifier Il to cancel the effect of the spurious or interfering signal,whereby the output of the channel amplifier li may be recorded in anundistorted form by the recorder 3. Y

A suitable filter or resistance network may be provided in each channelto adjust the intensity of the neutralizing signal to that of theinterfering spurious signal in that particular channel. This may beeffected for example by means of variable resistances 45, 41 and 49connected as a 'lf-network in the leads between the isolation amplifieri9 and the channel amplifier Il.

If desired, a single isolation amplifier may be used tol convey aneutralizing signal to all the channelsfof a sei-smic recording system.For this purpose, isolation amplifier and any further isolationamplifiers may be omitted, and the output of the isolation amplifier I0may be connected to the channel of detector 2 by closing a switch 55,- asimilar switch being used for all other channels, if any, of the seismicsystem. It is further obvious that instead of using a singleneutralizing unit to produce an inverse phase neutralizing signal fo:`all the channels of the seismic system, a separate neutralizing unit maybe used for each of the channels of said system, which can be eected,for example, by providing the transformer 9 with a plurality ofsecondarywindings or by using a plurality of such transformers.

I claim as my invention:

1. In a system for neutralizing interfering signals induced in a seismicrecording network by outside magnetic field oscillations, electroniccircuit means for converting said oscillations into an alternatingcurrent, means for passing a portion of thi-s current through a circuitcomprising in series a set of Helmholtz coils and an inductivereactance, means for passing another portion of this current through asecond circuit comprising in series a set of Helmholtz coils and acapacitive reactance, said sets of Helmholtz coils being arranged sothat their magnetic fields intersect at right angles, a coil rotatablypositioned at the intersection of said magnetic fields, means foradjusting the phase of the current induced in said coil when saidalternating current 'is passed through said Helmholtz coil'circuits,means for 4 amplifying said induced current, and means for introducingsaid amplified current into said seismic recording network to neutralizesaid interfering signals.

2. In a system for neutralizing interfering signals induced by outsidemagnetic field oscillations in a seismic recording system comprising aplurality of channels, electronic circuit means for converting saidoscillations into an alternating current, means for amplifying saidalternating current, means for passing a portion of said current througha circuit comprising in series an inductive resistance and a set ofHelmholtz coils,

means for passing another portion of said current through a secondcircuit comprising a capacitive reactance and a set of Helmholtz coils,said sets of Helmholtz coils being arranged so that their magneticfields intersect at right angles, a coil rotatably positioned at theintersection of said magnetic fields, means for adjusting the phase ofthe current induced in said coil when said alternating current is passedthrough said Helmholtz coil circuits, amplifying means connecting saidrotatable coil in parallel to each of the channels of the seismicrecording system, and means for adjusting the intensity of the amplifiedcurrent delivered by said amplifying means to .each of said channels.

3. In a system for neutralizing interfering signals induced in a seismicrecording network by outside magnetic field oscillations, meansindependent of said network for picking up said oscillations, means forconverting said oscillations into an alternating current, a firstcircuit comprising a set of Helmholtz coils energized by saidalternating current, a second circuit comprising a set of Helmholtzcoils energized by said alterhating current, said sets of Helmholtzcoils being positioned so that their magnetic fields intersect at rightangles, means in circuit with said Helmholtz coils to put the energizingcurrent owing in the first set of said coils in quadrature phaserelation with the current flowing in the second set of said coils, acoil rotatably positioned at the intersection of the magnetic fields ofsaid Helmholtz coils, means for adjusting the phase of the currentinduced in said rotatable coil by the energizing current flowing throughsaid Helmholtz coils, means for amplifying said induced current, andmeans for introducing said amplified current into said seismic recordingnetwork to neutralize the effect of said interfering outside fieldoscillations.

RONALD P. GILMORE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,543,668 Gewecke June 30, 1925'1,544,622 Affel July 7, 1925 1,630,346 Latour May 31, 1927 2,161,418Jones A z June 6, 1939 2,164,196 Woodyard et al. June 27, 1939 2,263,519Ritzmann Nov. 18, 1941 2,294,627 Parr Sept. 1, 1942 2,296,754 Wolf et alSept. 22, 1942 2,438,217 Johnson Mar. 23, 1948 FOREIGN PATENTS NumberCountry Date 427,796 Great Britain Apr. 26, 1935

